1. Field of the Invention
The present invention relates to a method for forming resist patterns in the fabrication of semiconductor devices, and more particularly, to a method for forming resist patterns having a vertical profile.
2. Description of the Prior Art
Where a resist pattern is formed using conventional single layer resist and tri-level resist methods, a notching occurs at side walls of the resist pattern due to light reflected by a substrate disposed under the resist pattern or an under layer. Such a notching results in a problem that the resist pattern has no vertical profile.
FIG. 1 illustrates the exposure of a resist film to light in accordance with a conventional method. As shown in FIG. 1, a photoresist film 34 is coated over a substrate or an under layer 35. As the photoresist film 34 is exposed to light 31 through a mask 32, the light transmitting the photoresist film 34 is reflected from the surface of the substrate 35 and then reflected again from the upper surface of the photoresist film 34. As a result, non-exposure portions of the photoresist film 34 are undesirably exposed to light 36 irregularly reflected as mentioned above. This results in a notching phenomenon that the side walls of a photoresist film pattern are partially removed.
Where a resist pattern is formed by coating a photoresist film over a substrate formed with another pattern and then patterning the photoresist film by use of exposure and development processes, a notching may occur due to the topology of the under pattern. In other words, the photoresist film pattern is partially removed at its side walls.
Generally, photoresist films used to obtain micropatterns are resist films having a low viscosity and a high sensitivity. Due to these properties, it is difficult for such photoresist films to have a large thickness. It is impossible to use photoresist films having a small thickness as masks for patterning metal layers. In order to increase the photoresist film thickness, a double resist coating method has been proposed.
FIG. 6 illustrates a conventional double resist coating method. In accordance with this method, a primary photoresist film 43 is formed over a silicon substrate 44 by coating a liquid phase photoresist material 46 through a nozzle 41 onto the silicon substrate 44 mounted on the shaft 45 of a spinner while rotating the silicon substrate 44, as shown in FIG. 6. A secondary photoresist film 42 is coated cover the primary photoresist film 43.
During the coating of the secondary photoresist film 42, the upper surface of the primary photoresist film 43 is dissolved by a solvent contained in the material for the secondary photoresist film 42, thereby causing an interface between the primary photoresist film 43 and the secondary photoresist film 42 to be poor. As a result, the primary and secondary photoresist films have a degraded uniformity in thickness. As the photoresist film thickness becomes larger, the resolution of the resulting resist pattern is degraded. This is because the undesirably large thickness of the photoresist film results in limiting the depth of focus which causes the process margin of the reduced exposure equipment to decline.